The wasilah (Arabic: وسيلة‎‎). is a means by which a person, goal or objective is approached, attained or achieved.

In another version of the meaning: Wasilah, according to Ibn Manzur (d. 711/1311) in Lisān al-'Arab means "a station with King, a rank, or act of devotion. In other words, it refers to a position of power due to one's proximity to the king or sovereign. While the tawassul or tawassulan is the use of wasilah for this purpose. http://sunnah.com/muslim/4/13

In religious contexts, the tawassul is the use of a wasilah to arrive at or obtain favour of Allah.

Two times word wasilah is used in Quran

[Maida 35]O you who have believed, fear Allah and seek the means [of nearness] to Him and strive in His cause that you may succeed.

[Isra 56-57]Say: "Call on those - besides Him - whom ye fancy: they have neither the power to remove your troubles from you nor to change them."

Those whom they call upon do desire (for themselves) means of access to their Lord, - even those who are nearest: they hope for His Mercy and fear His Wrath: for the Wrath of thy Lord is something to take heed of.

From above verses it is clear that the Wasilah in Maida-35 is in the form of command, i.e seek means that brings one closer to Allah. And Isra 56-57 says Wasilah (means of nearness) is not only restricted to humans and also certainly not in the manner which is negated by Allah there.

Most beloved things to Allah by which one gets closer to Him

Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "Allah said, 'I will declare war against him who shows hostility to a pious worshipper of Mine. And the most beloved things with which My slave comes nearer to Me, is what I have enjoined upon him; and My slave keeps on coming closer to Me through performing Nawafil (praying or doing extra deeds besides what is obligatory) till I love him, so I become his sense of hearing with which he hears, and his sense of sight with which he sees, and his hand with which he grips, and his leg with which he walks; and if he asks Me, I will give him, and if he asks My protection (Refuge), I will protect him; (i.e. give him My Refuge) and I do not hesitate to do anything as I hesitate to take the soul of the believer, for he hates death, and I hate to disappoint him."http://www.sunnah.com/bukhari/81/91

The best means by which seekers of nearness to Allah, the Glorified, the Exalted, seek nearness, is the belief in Him and His Prophet, fighting in His cause, for it is the high pinnacle of Islam, and (to believe) in the kalimatu'l-'ikhlas (the expression of Divine purification) for it is just nature and the establishment of prayer for it is (the basis of) community, payment of zakat (Islamic tax) for it is a compulsory obligation, fasting for the month of Ramadan for it is the shield against chastisement, the performance of hajj of the House of Allah (i. e . Ka`bah) and its `umrah (other than annual visit) for these two acts banish poverty and wash away sins, regard for kinship for it increases wealth and length of life, to giving alms secretly for it covers shortcomings, giving alms openly for it protects against a bad death and extending benefits (to people) for it saves from positions of disgrace.

Go ahead with the remembrance of Allah for it is the best remembrance, and long for that which He has promised to the pious, for His promise is the most true promise. Tread the course of the Prophet for it is the most distinguished course. Follow the sunnah of the Prophet for it is the most right of all behaviours. Learn the Qur'an for it is the fairest of discourses and understand it thoroughly for it is the best blossoming of hearts. Seek cure with its light for it is the cure for hearts. Recite it beautifully for it is the most beautiful narration. Certainly, a scholar who acts not according to his knowledge is like the off-headed ignorant who does not find relief from his ignorance, but on the learned the plea of Allah is greater and grief more incumbent, and he is more blameworthy before Allah.

[Shakir 5:35] O you who believe! be careful of (your duty to) Allah and seek means of nearness to Him and strive hard in His way that you may be successful.

[Pickthal 5:35] O ye who believe! Be mindful of your duty to Allah, and seek the way of approach unto Him, and strive in His way in order that ye may succeed.

[Yusufali 5:35] O ye who believe! Do your duty to Allah, seek the means of approach unto Him, and strive with might and main in his cause: that ye may prosper.
[Pooya/Ali Commentary 5:35]

Wasilah is a means of access to a thing or a being.

If Allah had willed He could have guided mankind directly through inspiration, but in His infinite wisdom, He had not deemed it desirable. He selected and appointed His representatives to convey His message and laws to people and administer their affairs in every age. Please refer to pages 1 to 7, and the commentary of al Baqarah: 2 to 5, 30 to 38, 48, and 124 to know that the Holy Prophet and his Ahl ul Bayt are the only means of approach to Allah.

Thus wasilah or means of access to Allah is to faithfully follow the Holy Prophet and the holy Imams of his holy house, who have been thoroughly purified by Allah (Ahzab: 33).

The Holy Prophet said:

"I and Ali are from one divine light."

"I will soon be called back, so I will have to go away from you, but I leave behind, amid you, the thaqalayn (two weighty indispensable influential authorities), the book of Allah and my Ahl ul Bayt. Should you be attached to these two, never, never shall you go astray, after me, for verily these two will never be separated from each other; and, joined together, they shall meet me at the spring of Kawthar."

"My Ahl ul Bayt amongst you are like the ark of Nuh. He who sails on it will be safe; but he who holds back shall be drowned and lost.".

[Shakir 17:56] Say: Call on those whom you assert besides Him, so they shall not control the removal of distress from you nor (its) transference.
[Pickthal 17:56] Say: Cry unto those (saints and angels) whom ye assume (to be gods) beside Him, yet they have no power to rid you of misfortune nor to change.
[Yusufali 17:56] Say: "Call on those - besides Him - whom ye fancy: they have neither the power to remove your troubles from you nor to change them."
[Pooya/Ali Commentary 17:56]

In addition to the false gods worshipped by the pagans, there were some who worshipped the angels as the daughters of God; and Uzayr and Isa are worshipped by the J ews and the Christians respectively as the sons of God.

[Shakir 17:57] Those whom they call upon, themselves seek the means of access to their Lord-- whoever of them is nearest-- and they hope for His mercy and fear His chastisement; surely the chastisement of your Lord is a thing to be cautious of.
[Pickthal 17:57] Those unto whom they cry seek the way of approach to their Lord, which of them shall be the nearest; they hope for His mercy and they fear His doom. Lo! the doom of thy Lord is to be shunned.
[Yusufali 17:57] Those whom they call upon do desire (for themselves) means of access to their Lord, - even those who are nearest: they hope for His Mercy and fear His Wrath: for the Wrath of thy Lord is something to take heed of.
[Pooya/Ali Commentary 17:57]

Aqa Mahdi Puya says:

It is futile to worship prophets, angels and heroes, inspite of the fact that they are good and holy and near to Allah, because even they seek means of access or approach (wasilah) to Allah.

Some schools of thought view that obedience itself is the wasilah, but verse 35 of al Ma-idah (see its commentary) does not agree with this point of view, as the believers are asked to seek wasilah besides taqwa, therefore obedience cannot be wasilah. The Quran and the Ahl ul Bayt teach us that the Holy Prophet's blessings and his prayer for forgiveness is a means of access or approach to Allah besides man's own deeds, because the Holy Prophet is the first and the foremost in the order of submission to and worship of Allah, being the seal of the prophets, and the superior-most among them. There are traditions, accepted by those scholars who do not follow the Ahl ul Bayt, which report that the companions of the Holy Prophet used to seek his prayers as the means of approach to Allah.

“O you who believe! be careful of (your duty to) Allah and seek means of nearness to Him and strive hard in His way that you may be successful.” (Qur’an, Surah Maidah 5:35)

And this verse has introduced three things that make one eligible for salvation and all three are found in Dua for His Eminence, the Master of the Time (aj) because: The first is the position of faith. And without any doubt Dua for His Eminence is a sign of faith and the cause if its perfection. And also: It is a type ofJihad by the tongue, in the same way it is a means towards the Beneficent Lord, which could be explained in two ways:

First: Meaning of Waseela (means) – as mentioned in Majmaul Bayan1 – is a relationship of connection and nearness, and there is no doubt that thisDua is a relationship of attachment and gaining nearness to Allah, the Mighty and the High, just as the other worship acts through which one can obtain the proximity of Allah. Although this Dua is one of the most important means of proximity and the nearest path of connection and the most valuable of them, as will be explained in this book by the help of Allah.

Second: It is that Waseela implies – especially in this blessed verse – that same Imam (as), as explained in the Tafseer of Ali Ibne Ibrahim Qummi:

“Seek His nearness through the Waseela of the Imam.”2

As evident, this statement is from a traditional report of Imam (as).

In Burhan it is narrated from Amirul Momineen (as) that he said regarding the verse:

“And seek means of nearness to Him.”

“I am the Waseela to Him.”3

In Miraat al-Anwaar quoting from the book of al-Waahida from Tariq bin Shahaab it is mentioned that he said: in a tradition, Amirul Momineen (as) said:

“Imams of Aale Muhammad (S) are Waseela to the Almighty Allah and the means of connection to His mercy…”4

Also in the book of Riyaaadh al-Jinaan it is narrated from Jabir that the Holy Prophet (S) mentioned the excellence of himself and his family when he said:

“We are Waseela to Allah.”5

In some Ziarats it is mentioned:

“And I make them a Waseela for gaining Your pleasure.”

It is mentioned in Dua Nudbah that:

“And I have made them source of gaining Your nearness and Waseela for obtaining Your satisfaction.”

In Dua of the Chief of the worshippers (as) on the Day of Arafah it is mentioned:

“And You made them Waseela to Yourself and a path towards Paradise.”6

From this we learn that Waseela denotes this same Imam. On the basis of this taking Waseela towards the Almighty Allah is performing an act that is source of satisfaction and nearness of His Eminence, and as the Almighty Allah has appointed a guide for every nation and an Imam for every people. He says:

“And (there is) a guide for every people.”

He appointed the Imam (as) as a Waseela towards Himself, thus it is necessary for every community to recognize their guide and Waseela and to do all that takes one near to him and earns his pleasure. Because without recognizing him, it would not be possible to become proximate to him.

From this point of view, it is mentioned in a tradition accepted by both sects (Shia and Sunni) that:

“One who dies without recognizing the Imam of his time, dies the death of Ignorance (infidelity).”7

On the basis of this, one who does not recognize the Imam of his time is like one who has not recognized any of the Imams and that which proves this matter are those widely related traditions, some of which we shall quote in the coming pages:

Husain (as) came to his companions and said: “O people, Allah, the Mighty and Sublime did not create human beings except that they should recognize Him. Thus if they recognize Him and (then) worship Him, they would become needless of worshipping anything else.” A man asked His Eminence: “O son of Allah’s Messenger (S), may my parents be sacrificed on you, what is the recognition of Allah?” He replied: “In every age the recognition (Marefat) of Allah is the recognition of the Imam whose obedience is obligatory on the people.”8

Here the author of this book quotes from his teacher, Allamah Majlisi an anecdote that the latter has mentioned in Biharul Anwar:

From this point of view the recognition (Marefat) of Allah is interpreted as theMarefat of Imam (as) perhaps because the Marefat of Allah cannot be gained except through Imam or that getting a share of divine recognition (Marefat) is subject to Marefat of Imam (as).........

[Pickthal 4:64] We sent no messenger save that he should be obeyed by Allah's leave. And if, when they had wronged themselves, they had but come unto thee and asked forgiveness of Allah, and asked forgiveness of the messenger, they would have found Allah Forgiving, Merciful.
[Pooya/Ali Commentary 4:64]

It is the will of Allah that people should obey the Holy Prophet. If the sinners approach the Holy Prophet, he will speak on their behalf to Allah, because He has given him the authority to intercede on behalf of the repentant sinners; and on his (and his Ahl ul Bayt's) recommendation. Allah forgives the sinners, as has been clearly mentioned in this verse. Refer to the commentary of al-Baqarah: 48.